Electric power steering apparatus

ABSTRACT

An electric power steering apparatus including a reduction mechanism  5  for transmitting an output of a steering assist motor  1  to a steering mechanism through a worm  3  and a worm wheel  4 , in which the worm wheel  4  is formed by a thermosetting synthetic resin filled with a sheet  6  of aramid fibers having a low attacking property to a counter member and an effect of improving the abrasion resistance of the synthetic resin. Even when the reduction mechanism is used in an engine room or the temperature of the reduction mechanism is raised by frictional heat with heightening power, the reduction mechanism is durable, and further the gears have excellent abrasion resistance and toughness and improved durability.

This application is a divisional of application Ser. No. 09/915,274,filed on Jul. 27, 2001, now U.S. Pat. No. 6,557,663 the entire contentsof which are hereby incorporated by reference and for which priority isclaimed under 35 U.S.C. § 120; and this application claims priority ofApplication No. 2000-227890 filed in Japan on Jul. 27, 2000 under 35U.S.C. § 119.

BACKGROUND OF THE INVENTION

The present invention relates to an electric power steering apparatususing a motor as the source of steering assist force.

FIG. 1 is a cross sectional view showing the structures of aconventional electric power steering apparatus.

As shown in FIG. 1, for example, an electric power steering apparatusfor automobiles comprises a first steering shaft 101 connected to asteering wheel 100 for steering; a second steering shaft 103 whose upperend portion is connected coaxilally with the lower end portion of thefirst steering shaft 101 through a torsion bar 102 and whose lower endportion is connected to a steering mechanism joined to wheels; a torquesensor 104 for detecting a torque applied to the first steering shaft101 by rotation of the steering wheel 100, based on twist of the torsionbar 102; a steering assist motor driven according to the result ofdetection given by at least the torque sensor 104; and a reductionmechanism which is joined to an output shaft of the motor and includes aworm 105 and a worm wheel 106 for decelerating rotation of the outputshaft and transmitting the resultant rotation to the second steeringshaft 103. Such an electric power steering apparatus for automobiles isconstructed to reduce the driver's labor for steering by assisting theoperation of the steering mechanism according to the rotation of thesteering wheel 100 with the rotation of the motor.

The worm 105 constituting the reduction mechanism is positioned to crossthe axis of the second steering shaft 103 and supported in a housing 107through a pair of roller bearings. The second steering shaft 103 towhich the worm wheel 106 is attached is supported in the housing 107through a pair of roller bearings 108.

The worm wheel 106 includes a circular tooth member which is made of asynthetic resin and has teeth meshing with the worm 105, and a metalfitting member to be fitted into the circular tooth member. The wormwheel 106 is made of a synthetic resin, so that noise caused by meshingwith the worm 105 is reduced and the workability in making the teeth isimproved.

An electric power steering apparatus using a motor as the source ofsteering assist force as mentioned above is employed mainly by midgetcars and relatively small cars with an engine displacement of around1300 cc, for example. On the other hand, relatively large cars with anengine displacement of around 2000 cc or more, for example, often employa hydraulic steering apparatus using a hydraulic pump connected to theengine as the source of steering assist force.

By the way, since an engine as a travel driving source is also used as ahydraulic pump driving source of the hydraulic steering apparatus, evenif steering assistance is not intended, the hydraulic pump is driven bythe engine, and thus hydraulic steering apparatus does not meet a demandfor low fuel consumption. Moreover, there is an increasing demand forapplying en electric power steering apparatus to cars with an enginedisplacement of around 1300 cc to 2000 cc. According to this demand, ithas been considered to apply the electric power steering apparatus torelatively large and heavy vehicles or tracks in the near future, andthus steering assistance with higher power compared to a conventionalsteering assist force will be required. Accordingly, in the case whereat least a part of the worm and worm wheel constituting the reductionmechanism is formed by a synthetic resin, the high-heat-resistant,high-strength worm and worm wheel are necessary.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide an electric powersteering apparatus capable of solving the above problem.

An electric power steering apparatus according to the first invention isan electric power steering apparatus for assisting steering bytransmitting an output of a steering assist motor to a steeringmechanism through a driving gear and a driven gear meshing with thedriving gear, and characterized in that at least one of the driving gearand driven gear is formed by a thermosetting synthetic resin filled witharamid fibers.

According to the first invention, since one or both of the driving gearand driven gear are formed by a thermosetting synthetic resin with anexcellent heat resistance, even when the reduction mechanism is used inan engine room or the temperature of the reduction mechanism is raisedby frictional heat with heightening power, the gear is durable.Moreover, the thermosetting synthetic resin is filled with aramid fibershaving a low attacking property to a counter member and an effect ofimproving the abrasion resistance of the synthetic resin, one or both ofthe driving gear and driven gear filled with the aramid fibers has anexcellent abrasion resistance and toughness and further enables areduction in the abrasion of the gear as the meshing counterpart,thereby improving the durability of both the gears. Furthermore, sincethe aramid fibers have a small thermal expansion coefficient and goodheat stability, the gears can have excellent dimensional stability. Inparticular, since a para-linked aramid fiber has a negative thermalexpansion coefficient, it is convenient to limit the thermal expansionof the thermosetting synthetic resin. However, since the para-linkedaramid fiber is extremely tough, it is difficult to form the teeth ofthe gear by cutting. Therefore, it is preferred to mix the para-linkedaramid fibers with meta-linked aramid fibers that have slightly lowertoughness but have good workability and to fill the mixed fibers.

An electric power steering apparatus according to the second inventionis characterized in that the aramid fibers are formed into a sheet.Here, the sheet is, for example, felt obtained by needling a pile of thearamid fibers to couple the respective fibers, woven material or knittedmaterial made by using threads of the aramid fibers. The needling is aprocess of putting a needle having a hook into the fiber pile anddrawing the needle out of the fiber pile in the direction of thethickness of the fiber pile so that the fibers caught by the hook inthis action are oriented in the thickness direction. The fibers orientedin the thickness direction perform the function of coupling the fibersoriented in the plane direction.

According to the second invention, since the positional arrangement ofthe aramid fibers and the distance between the aramid fibers can befreely set, it is possible to further increase the abrasion resistanceand toughness of the gear filled with the sheet.

An electric power steering apparatus according to the third invention ischaracterized in that the sheet is wound repeatedly into a cylindricalshape and further folded like bellows in an axial direction of thecylindrical shape.

According to the third invention, since the sheet of the aramid fibersare wound repeatedly into the cylindrical shape and folded in the axialdirection of the cylindrical shape, the aramid fibers form circularfiber layers and are three-dimensionally positioned. Therefore, evenwhen the teeth are formed by cutting, it is possible to further increasethe abrasion resistance and toughness of the tooth portion.

When this structure is applied particularly to the worm wheel byselecting the above-mentioned felt as the sheet, the contact faces ofthe folds of the sheet are stacked in the axial direction, so that theshear force exerted from the worm is spread and received evenly by therespective contact faces. It is therefore possible to further improvethe strength and durability.

On the other hand, when a structure where the sheet is simply woundrepeatedly is adopted, the contact faces of the wound sheet are stackedin a radial direction, so that the direction of the shear force exertedfrom the worm and the direction of the contact faces agree with eachother. Therefore, a separation phenomenon tends to occur at the contactfaces, and thus it is not preferred to adopt such a structure.

An electric power steering apparatus according to the fourth inventionis characterized in that the sheets are stacked in layers.

According to the fourth invention, since sheets of the aramid fibers arelayered to form a plurality of fiber layers, it is possible to equalizethe density of the aramid fibers, thereby further increasing theabrasion resistance and toughness of the tooth portion even when theteeth are formed by cutting.

An electric power steering apparatus according to the fifth invention ischaracterized in that the aramid fibers are filled in a ratio of 20 to60% by volume.

According to the fifth invention, since the aramid fibers filled in aratio of 20 to 60% by volume reinforce the thermosetting syntheticresin, one or both of the driving gear and driven gear have excellentabrasion resistance and toughness and improved durability.

Further, when the filling amount of the aramid fibers is less than 20%by volume, the brittleness as a characteristic of the thermosettingsynthetic resin is increased, and consequently the thermosettingsynthetic resin can not be satisfactorily reinforced. Thus, it is notpreferred to fill the aramid fibers in an amount less than 20% byvolume. On the other hand, when the filling amount of the aramid fibersexceeds 60% by volume, it is impossible to sufficiently fill thethermosetting synthetic resin into the respective fibers, resulting ininsufficient reinforcement of the thermosetting synthetic resin by thearamid fibers. Therefore, it is not preferred to fill the aramid fibersin an amount more than 60% by volume.

The above and further objects and features of the invention will morefully be apparent from the following detailed description withaccompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross sectional view showing the structures of aconventional electric power steering apparatus;

FIG. 2 is a cross sectional view showing the structures of a reductionmechanism and a portion around a motor of an electric power steeringapparatus according to the present invention;

FIG. 3 is an enlarged cross sectional view showing a worm wheel of theelectric power steering apparatus according to the present invention;

FIG. 4 is a cross sectional view cut along the III—III line of FIG. 3;

FIG. 5 is a perspective view of a sheet for use in the worm wheel of theelectric power steering apparatus according to the present invention;

FIG. 6 is an explanatory view illustrating the process of forming thesheet in a ring shape for use in the worm wheel of the electric powersteering apparatus according to the present invention;

FIG. 7A is a perspective view showing a state in which a sheet of aramidfiber is deformed in a ring shape for use in the worm wheel of theelectric power steering apparatus according to the present invention;

FIG. 7B is a perspective view of the sheet cut along the A-A′ line;

FIG. 8A is an enlarged plan view of a sheet showing the structures ofthe second embodiment of the electric power steering apparatus accordingto the present invention; and

FIG. 8B is a cross sectional view of the same sheet.

DETAILED DESCRIPTION OF THE INVENTION

The following description will explain the present invention withreference to the drawings illustrating some embodiments thereof.

First Embodiment

FIG. 2 is a cross sectional view showing the structures of a reductionmechanism and a portion around a motor of an electric power steeringapparatus according to the present invention; FIG. 3 is an enlargedcross sectional view of a worm wheel; and FIG. 4 is a cross sectionalview cut along the III—III line of FIG. 3.

Since the basic structures of the electric power steering apparatus aresimilar to those of a conventional electric power steering apparatusshown in FIG. 1, the detailed explanation of the similar structures andthe explanation of the functions will be omitted.

A reduction mechanism 5 comprises a metal worm 3 which is joined to anoutput shaft 1 a of a steering assist motor 1 and positioned to crossthe axis of a second steering shaft 2; and a worm wheel 4 which mesheswith the worm 3 and is fitted and fixed on the middle of the secondsteering shaft 2. The worm wheel 4 of the reduction mechanism 5 includesa circular tooth member 41 which is made of a synthetic resin and has aplurality of teeth 4 a to mesh with the worm 3; and a fitting member 42formed by a metal to be fitted inside the circular tooth member 41.Further, a through hole 42 a formed at the center of this fitting member42 is fitted on the second steering shaft 2.

FIG. 5 is a perspective view showing an example of a sheet of aramidfibers for use in the worm wheel 4. This sheet 6 is in the shape of feltformed by needling a pile of fibers (with a fiber length of around 50mm) obtained by mixing para-linked aramid fibers with meta-linked aramidfibers, and fibers 61 oriented in the thickness direction of the fiberpile couple fibers 62 oriented in the plane direction by the needling.In this embodiment, “Technora®” available from TEIJIN LIMITED is used asthe para-linked aramid fibers and “TEIJINCONEX®” available from TEIJINLIMITED is used as the meta-linked aramid fibers, and they are eachmixed in a ratio of 50% by mass.

FIG. 6 is an explanatory view illustrating the process of deforming thearamid fiber sheet in the shape of felt into a ring shape. FIG. 7A is aperspective view showing a state in which the aramid fiber sheet isdeformed in a ring shape by the process shown in FIG. 6, and FIG. 7B isa perspective view of the sheet cut along the A-A′ line.

As shown in FIG. 6, this sheet 6, i.e., the aramid fiber felt, is woundrepeatedly to form a cylindrical shape and pressed in an axial directionof the cylindrical shape within a die for temporary forming so that itis folded like bellows and further formed into a ring shape with thebellows being folded as shown in FIGS. 7A and 7B. The sheet 6 has a sizethat allows formation of a non-tooth portion and a tooth portion wherethe teeth 4 a are formed.

The sheet 6 thus formed in a ring shape is stored and positioned in acompression-molding die or casting-molding die, and a thermosettingsynthetic resin, such as phenol resin, melamine resin, cross-linkedpolyester amide or epoxy resin, is molded by compression molding orinjection molding.

In other words, in compression molding, the circular sheet 6 carryingthe thermosetting synthetic resin, such as phenol resin or melamineresin, and the above-mentioned fitting member 42 are stored andpositioned in the compression-molding die. Then, by applying heat andpressure, the melted thermosetting synthetic resin sufficientlypermeates between the respective aramid fibers, and consequently thecircular tooth member 41 having the sheet 6 even in the portion wherethe teeth are to be processed is formed and the fitting member 42 iscoupled to the circular tooth member 41 in one body.

On the other hand, in casting molding, the ring-shaped sheet 6 andfitting member 42 are stored and positioned in the casting-molding diehaving an air suction gate and a resin filling gate. Then, when the airin the casting-molding die is removed from the air suction gate to makea vacuum (when the pressure is reduced), the melted thermosettingsynthetic resin, such as cross-linked polyester amide or epoxy resin, isintroduced into the casting-molding die from the resin filling gate dueto the negative pressure, the thermosetting synthetic resin liquidsufficiently permeates between the respective aramid fibers, thecircular tooth member 41 having the sheet 6 even in the portion wherethe teeth are to be processed is formed, and the fitting member 42 iscoupled to the circular tooth member 41 in one body.

In this compression molding or casting molding, since the thermosettingsynthetic resin has higher fluidity in a melted state as compared with athermoplastic synthetic resin, even when the sheet 6 of the aramidfibers is formed in a ring shape, it is possible to satisfactorilypermeate the thermosetting synthetic resin between the respective aramidfibers.

The aramid fibers are filled in a ratio of 20 to 60% by volume. When thefilling amount of the aramid fibers is smaller than 20% by volume, thebrittleness as a characteristic of the thermosetting synthetic resin isincreased, and consequently the thermosetting synthetic resin can not besatisfactorily reinforced. On the other hand, when the filling amount ofthe aramid fibers exceeds 60% by volume, it is impossible tosufficiently permeate (fill) the thermosetting synthetic resin betweenthe respective fibers during the above-mentioned molding, resulting ininsufficient reinforcement of the thermosetting synthetic resin by thearamid fibers.

In the fitting portion between the circular tooth member 41 and thefitting member 42, i.e., the outside of the fitting member 42, raisedand recessed lines 43 such as splines are formed at the center in theaxial direction of the fitting member 42 and circular recesses 44 areformed at both end portions in the axial direction. Raised and recessedlines and circular protrusions to be fitted into the raised and recessedlines 43 and circular recesses 44 are formed on the inner side of thecircular tooth member 41, so that the circular tooth member 41 and thefitting member 42 are coupled together in such a manner that relativerotation and relative movement in the axial direction are infeasible.Alternatively, the circular tooth member 41 and the fitting member 42may be coupled together in such a manner that relative rotation andrelative movement in the axial direction are infeasible by providingraised and recessed lines such as knurls on the outer side of thefitting member 42 at the fitting portion between the circular toothmember 41 and fitting member 42.

After molding in the above-mentioned manner, the teeth 4 a with a toothlead twisting in a rotation direction with respect to a rotation axisare formed on the peripheral surface of the circular tooth member 41 bycutting.

Further, in the first embodiment, the sheet 6 of the aramid fibers maybe arranged such that the flat sheet is wound into a stick-like shapelike a twisted-paper string and this stick is made a circular shape withits both end portions partly overlapped to finally form a ring.

Second Embodiment

FIG. 8A is an enlarged plan view of a sheet of the aramid fibers showingthe structures of the second embodiment, and FIG. 8B is a crosssectional view of the same sheet of the aramid fibers.

In this second embodiment, a plurality of sheets 6 of woven aramidfibers are layered, and the layered sheets are compressed in thelayering direction by a relatively small force and then punched in aring shape by a press.

Like the first embodiment, the sheet 6 of this second embodiment issubjected to compression molding or casting molding so that the sheet 6and thermosetting synthetic resin are made in one body and form thecircular tooth member 41.

Since other structures and functions are the same as those of the firstembodiment, the detailed explanation of the structures and theexplanation of the functions will be omitted.

Third Embodiment

In this third embodiment, instead of forming the sheet 6 of aramidfibers by weaving the aramid fibers, a felt similar to that used in thefirst embodiment is used and, like the second embodiment, a plurality ofthis felt sheets are layered, the layered sheets are compressed in thelayering direction by a relatively small force and then punched in acircular shape by a press.

Like the first embodiment, the sheet 6 (felt) of the aramid fibers ofthis third embodiment is subjected to compression molding or castingmolding so that the sheet 6 and the thermosetting synthetic resin aremade in one body and form the circular tooth member 41.

Since other structures and functions are the same as those of the firstembodiment, the detailed explanation of the structures and theexplanation of the functions will be omitted.

Fourth Embodiment

In this fourth embodiment, the circular tooth member 41 is formed byinjection molding using a granular compound obtained by mixing andkneading a thermosetting synthetic resin and aramid fibers having afiber length longer than 400 μm and pulverizing the mixture, instead ofknitting or weaving threads of the aramid fibers or accumulating thearamid fibers to make felt.

In the fourth embodiment, the fitting member 42 is stored and positionedas an insert in the injection-molding die, and the granular compound ismelted and then injected to the periphery of the fitting member 42. Theteeth 4 a are formed by molding, without cutting. In the injectionmolding, a satisfactory molding result is not obtained unless the amountof the aramid fibers is made 30% by volume or less. The reason for thisis that if the amount of the aramid fibers exceeds 30% by volume, thearamid fibers twist round a gate and obstruct the injection.

When the fiber length of the aramid fibers is 400 μm or less, thebrittleness as a characteristic of the thermosetting synthetic resin isincreased, and consequently the thermosetting synthetic resin can not besatisfactorily reinforced. In the fourth embodiment, however, since thearamid fibers have a relatively long fiber length of more than 400 μm,the worm wheel 4 has an excellent abrasion resistance and toughness andfurther enables a reduction in the abrasion of the worm 3 as the meshingcounterpart, thereby improving the durability of the worm 3 and wormwheel 4. Furthermore, when the fiber length of the aramid fibers exceeds8 mm, the aramid fibers are cut to a length less than 8 mm by aninjection screw during injection molding. In other words, in injectionmolding, since the aramid fibers can not have a length longer than 8 mm,it is not necessary to use aramid fibers with a fiber length longer than8 mm.

Note that while the sheet 6 is formed by the aramid fibers in theabove-explained first embodiment, this sheet 6 may also be formed byfibers made from heat-resistant, high-strength organic macromolecularpolymers such as heterocycle-containing aromatic polymer or polyetherether ketone (PEEK). Moreover, it is also possible to mix and use glassfibers, carbon fibers, etc. within a range in which the counter memberattacking property is not noticeable.

Besides, in the above-explained first through fourth embodiments, whilethe circular tooth member 41 of the worm wheel 4 constructed by couplingthe fitting member 42 with the circular tooth member 41 is formed by thethermosetting synthetic resin filled with the aramid fibers, it ispossible to form the entire worm wheel 4 or the worm 3 by thethermosetting synthetic resin filled with the aramid fibers.Alternatively, for example, it is possible to employ a structure inwhich the tooth portion has the aramid fibers and the non-tooth portionhas glass fibers or carbon fibers.

Moreover, the reduction mechanism 5 in the above-explained first throughfourth embodiments may be a hypoid gear including a driving gear as ahypoid pinion and a driven gear as a hypoid wheel, instead of a wormgear including a driving gear 3 as a worm and a driven gear 4 as a wormwheel. Further, the reduction mechanism may be a bevel gear.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiments are therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within metesand bounds of the claims, or equivalence of such metes and boundsthereof are therefore intended to be embraced by the claims.

What is claimed is:
 1. A method of manufacturing at least one of thedriving gear or a driven gear for an electric powered steering apparatusfor assisting steering by transmitting an output of a steering assistmotor to a steering mechanism through said driving gear and said drivengear meshing with said driving gear, the method comprising the steps of:forming aramid fibers into a sheet; wounding said sheet repeatedly in acylindrical shape; folding said wound sheet in an axial direction of thecylindrical shape, forming bellows; and molding said aramid fibers witha thermosetting synthetic resin, wherein said aramid fibers are filledin a ratio of 20 to 60% by volume.
 2. The method according to claim 1,further comprising the steps of molding said sheets by compressionmolding or casting molding.
 3. The method according to claim 1, furthercomprising the steps of forming teeth by cutting the thermosettingsynthetic resin and aramid fibers mixture.
 4. The method according toclaim 1, wherein said sheet is obtained by needling said aramid fibers.5. A method of manufacturing at least one of the driving gear or adriven gear for an electric powered steering apparatus for assistingsteering by transmitting an output of a steering assist motor to asteering mechanism through said driving gear and said driven gearmeshing with said driving gear, the method comprising the steps of:forming aramid fibers into a sheet; stacking said sheet into layers;compressing said layered sheets in the layering direction; and moldingsaid aramid fibers with a thermosetting synthetic resin, wherein saidaramid fibers are filled in a ratio of 20 to 60% by volume.
 6. Themethod according to claim 5, further comprising the steps of moldingsaid sheets by compression molding or casting molding.
 7. The methodaccording to claim 5, further comprising the steps of forming teeth bycutting the thermosetting synthetic resin and aramid fibers mixture. 8.The method according to claim 5, wherein said sheet is obtained byweaving said aramid fibers.
 9. The method according to claim 5, whereinsaid sheet is obtained by needling said aramid fibers.
 10. The methodaccording to claim 5, further comprising the step of punching saidcomposed layered sheets into a circular shape by a press.